explicit specialization of 'is_string<std::FILE *>' after instantiation

[riccozzz]

Example here. I believe per commit c21c6b8

This error will happen when you include format.h, format with a std::FILE* handle, and later include color.h (ie from another translation unit).

Specialization happens here:

fmt/include/fmt/color.h

Lines 521 to 526 in 02a6f16

	// The following specialization disables using std::FILE as a character type,
	// which is needed because or else
	// fmt::print(stderr, fmt::emphasis::bold, "");
	// would take stderr (a std::FILE *) as the format string.
	template <> struct is_string<std::FILE*> : std::false_type {};
	template <> struct is_string<const std::FILE*> : std::false_type {};

Can these be forward declared in core.h before instantiation?